The present invention relates to a method for fabricating a semiconductor device using a nitride semiconductor which functions as a short-wavelength light-emitting diode, a short-wavelength semiconductor laser, a high-temperature and high-speed transistor, or the like.
A nitride semiconductor which is large in optical band gap (e.g., GaN has an optical band gap of about 3.4 eV at a room temperature) has been used conventionally as a material for implementing a visible-range light-emitting diode which emits light in a relatively short wavelength region such as green, blue, or white light or a short-wavelength semiconductor laser which is effective in increasing the capacity of an optical disk. In particular, a nitride semiconductor has been used prevalently for the active layer of a light-emitting diode. As a light source for a read/write operation to a high-density optical disk, the commercialization of a blue or blue-purple laser has been in strong demand.
As a background to the increasing commercial availability and mass producibility of these devices, there have been several technological breakthroughs, one of which is advances in heteroepitaxial growth technology represented by the introduction of a low-temperature buffer layer. If a GaN layer is used, it is necessary to perform crystal growth on a substrate of a different material since a bulk GaN substrate does not exist. Under the circumstances, a method of epitaxially growing the GaN layer by metal organic chemical vapor deposition (MOCVD) on a sapphire substrate has been used widely because the sapphire substrate has a hexagonal structure, similarly to the GaN substrate. In an example of the method, an amorphous AlN layer or a GaN low-temperature buffer layer is formed on a sapphire substrate and then an epitaxially grown layer which is a group III-V compound semiconductor layer forming a principal portion of a device is formed by CVD at a relatively high temperature so that a semiconductor layer having a flat surface and a reduced number crystal defects is obtained.
On the other hand, significant progress has also been made in improving device structures, elucidating the physical phenomenon of crystal growth, or developing a technique for growing a mixed crystal such as InGaN or AlGaN.
Another breakthrough is the implementation of a low-resistance p-type layer. It was previously difficult to implement a p-type GaN layer having a low resistance in an epitaxially grown layer even if the p-type GaN layer is doped with Mg, which is a group II element, as a dopant. However, it has been proved recently that, if annealing is performed with the application of an electron beam or in a nitrogen atmosphere after the formation of the epitaxially grown layer, the p-type GaN layer can be reduced in resistance. It has also been proved that the mechanism of the reduced resistance of the p-type GaN layer is the removal of hydrogen from the p-type GaN layer since impurity atoms are passivated with hydrogen in the p-type GaN layer.
The two breakthroughs described above have allowed a pn junction with an excellent crystalline property to be obtained with high reproducibility so that a light-emitting diode using this has been commercialized and a semiconductor laser using this is close to commercialization.
A description will be given to a method for fabricating the aforementioned nitride semiconductor device. FIGS. 9A to 9C are cross-sectional views illustrating a conventional method for fabricating a nitride semiconductor device.
First, in the step shown in FIG. 9A, an n-type InGaAlN layer 104 having a thickness of about 2 μm and a composition represented by (AlxGa1-x)yIn1-yN (0≦x≦1, 0≦y≦1) is formed on a sapphire substrate 101 (wafer) by, e.g., metal organic chemical vapor deposition (MOCVD). The n-type InGaAs layer may also be formed after an amorphous AlN buffer layer as thin as about 50 nm is formed at a low temperature of, e.g., about 500° C. The n-type InGaAlN layer 104 includes an n-type GaN layer or an n-type AlGaN clad layer, though it is not depicted. Subsequently, an undoped InGaAlN active layer 103 having a composition represented by (AlxGa1-x)yIn1-yN (0≦x≦1, 0≦y≦1) is formed on the n-type InGaAlN layer 104. The InGaAlN active layer 103 contains, e.g., an InGaN quantum well structure and serves as a region emitting blue or blue-purple light in response to the injection of a current if the semiconductor device is a light-emitting diode or a semiconductor laser. Subsequently, a p-type InGaAlN layer 102 having a thickness of about 2 μm and a composition represented by (AlxGa1-x)yIn1-yN (0≦x≦1, 0≦y≦1) is formed on the InGaAlN active layer 103. The p-type InGaAlN layer 102 includes a p-type AlGaN clad layer or a p-type GaN layer. Further, an oxide film cap layer 106 composed of a silicon dioxide is formed by CVD on the p-type InGaAlN layer 102.
In the step shown in FIG. 9A, the formation of the p-type InGaAlN layer 102 uses, e.g., Cp2Mg so that the p-type InGaAlN layer 102 has been doped with Mg. In the as-grown state, Mg atoms are passivated with hydrogen atoms in the p-type InGaAlN layer 102 so that the p-type InGaAlN layer 102 has a high electric resistance. For the removal of hydrogen from the p-type InGaAlN layer 102, it is normally necessary to perform a heat treatment in a gas atmosphere not containing hydrogen.
Next, in the step shown in FIG. 9B, the wafer is retrieved from a crystal growing apparatus and placed in a furnace containing a nitrogen gas atmosphere, e.g., a lamp heat furnace. Then, a heat treatment at a temperature of, e.g., about 700° C. is performed with respect to the wafer by using a heating lamp 10, whereby the p-type InGaAlN layer 102 is reduced in resistance.
Next, in the step shown in FIG. 9C, the oxide film cap layer 106 is removed. Thereafter, a semiconductor laser, a light-emitting diode, or the like is formed by using the p-type InGaAlN layer 102, the InGaAlN active layer 103, and the n-type InGaAlN layer 104.
However, the foregoing method for fabricating a nitride semiconductor has the following problems.
For sufficient activation of the p-type impurity in the p-type InGaAlN layer 102 shown in FIG. 9B, a high temperature on the order of 800° C. is needed. However, since the temperature is equal to a temperature at which the InGaAlN active layer 103 is grown, the degradation of the InGaAlN active layer 103, such as the diffusion of In atoms in the quantum well structure of the InGaAlN active layer 103, may occur. If the temperature for the heat treatment is reduced for the prevention of the degradation, the resistivity of the p-type InGaAlN layer 102 is increased disadvantageously, which causes the problems that de resistances between the p-type InGaAlN layer 102, the InGaAlN active layer 103, and the n-type InGaAlN layer 104 and the contact resistance of an electrode connected to the InGaAlN layer are increased. Briefly, it has been difficult to simultaneously perform the formation of the active layer at a low temperature and the reduction of the resistance of the p-type InGaAlN layer.